This invention relates generally to the maintenance of batteries. More particularly, this invention pertains to an apparatus and method for enhancing the life of rechargeable lead-acid storage batteries and indicating when the battery requires servicing or replacement.
A conventional lead-acid storage battery of the type used for motor vehicles often fails when not used frequently. Thus, for example, while automobile lead-acid batteries under frequent use may typically last 4-6 years, the same batteries used in boats often fail after storage over a single winter season, even when kept fully charged. It is known that during periods of inactivity, excessive quantities of lead sulphate form on the internal plates. The solid lead sulphate acts as an effective electrical insulator to prevent high current densities in either a discharge or charge mode. In addition, the particle size of the precipitated lead sulphate may be so large that complete reversal to lead or lead oxide and sulphuric acid does not occur upon recharging. Thus, the battery may be useless for high current uses such as starting engines, etc.
Failure of batteries during storage has also been partially attributed to the storage conditions. Typically, the battery is stored on a cold basement or garage floor. Even though the battery is kept fully charged, the internal temperature difference, from top to bottom, is exacerbated by standing on the cold floor, and may result in rapid battery deterioration.
Complete or nearly-complete discharge of lead-acid batteries often leads to an early demise. In addition, excess discharge in a cold climate may lead to freezing of the electrolyte, with attendant cracking of the electrodes and battery case.
The propensity to battery failure is not simply a result of excessive discharge. Premature failure often results even when the battery is maintained at a high charge level with a "trickle charger" for example, and is not permitted to be deeply discharged. The premature failure is due to excessive accumulation of lead sulphate.
In many situations, storage batteries are subjected to long periods of inactivity. Examples of such include 24 volt battery banks for starting emergency generators in hospitals, defense works, remote microwave communications installations, remote water and sewage pumps, and the like. Battery failure in such instances may result in loss of property and/or communications, or even life, for example, in a hospital situation.
Similarly, batteries used for starting boat and motorcycle motors, or operating small trolling motors, typically have a very short life. The short life is often attributed to long term winter storage without frequent charging, but premature battery failure is well known to occur even when the battery charge is maintained.
Any method for extending the life of lead-acid batteries will not only reduce the cost of replacement, but will reduce secondary costs related to battery failure.
The conventional method for testing a battery is to determine whether the battery will accept and maintain a full charge voltage for a short period of time. Thus, a nominal 12 volt lead-acid battery may be considered to be in acceptable condition if it will accept a charge of e.g. about 11 to 12.5 volts and no significant drop in voltage occurs upon standing for a period of several hours following the charging cycle.
It has been found that the propensity to fail is not generally detectable by measuring the voltage across the fully charged battery, presumably because such no-load voltage is not significantly related to the degree of lead sulphate plating with its resulting increase in internal resistance. Even though commonly used, the inadequacy of this test is well-known and calls for an improved test procedure, particulary where the intended useful current load on the battery may be relatively high.
Lead-acid batteries are typically rated in several ways. First, the term "cold-cranking amperage" refers to the battery capacity, in ampere-hours, when discharged at a rate which will draw it down to a given cell voltage in five minutes.
However, the most common measurement of storage battery capacity is the five-hour rating, obtained by discharging the battery at a rate which will discharge it in five hours. The five-hour rating will always be much higher than the five-minute rating which more closely approximates the use of the battery in an engine starting situation.
The term "reserve capacity" refers to the additional capacity in amperes at which a battery potential of 10.5 volts is maintained. In this rating, time is not strictly taken into account. Obviously, if even a small load is placed on a battery, it will in time drop below 10.5 volts, and will eventually be entirely depleted, i.e. zero volts potential.
An apparatus and method for both extending battery life and determining when the battery is in an impending failure mode will greatly increase the effectiveness, reliability and safety of lead-acid batteries.